1. Field of the Invention
The invention relates to the field of production printing systems and, in particular, to calibrating printers that print to a plurality of different print mediums, such as different types of paper.
2. Statement of the Problem
Production printing systems associated with data processing enterprises generally include a localized print controller within the printing system. The print controller controls the overall operation of the printing system including, for example, host interfacing, interpretation or rendering, and lower level process control or interface features of print engines of the printing system. Host interaction may include appropriate adapters for coupling the printing system to one or more host systems that transmit print jobs to the printing system. The print jobs are generally encoded in the form of a page description language such as PostScript (PS), PCL, IPDS, etc.
In whatever form the print job may be encoded or formatted, the print controller within the printing system interprets the received information to generate sheetside bitmaps of the print job. The sheetside bitmaps represent the image to be printed on one side of a sheet of a print medium. Each sheetside bitmap generally comprises a 2-dimensional array of picture elements (“pixels”, or PELs) that represent a corresponding formatted sheet of the print job. Each pixel may represent an encoded color value in accordance with the requirements of the particular print job encoding and the capabilities of the printing system on which the print job is to be printed.
The print controller stores or buffers the sheetside bitmaps in accordance with storage capabilities of the particular architecture of a particular print controller. The print controller then forwards the sheetside bitmaps to one or more print engines (sometimes also referred to as an “imaging engine” or as a “marking engine”). The print engines have internal queues for storing the sheetside bitmaps to be printed. A print engine pulls the sheetside bitmaps off of the queue and performs an imaging process to mark the print medium with the sheetside bitmaps provided by the print controller. The print engine may comprise a laser print engine, an ink jetprint engine, or another type of imaging system that transfers each sheetside bitmap to corresponding pixels on paper. Generally, the print engine is configured with the printer.
Output quality for the printers generally depends on the print engine characteristics being known and fixed, so that the color conversions and transfer curves can be constructed in advance. This known state may be referred to as the reference state. In practice, print engines tend to become uncalibrated due to environmental conditions and operating conditions. This “printer drift” degrades the output quality of a printed product because the amount of deposited toner varies. And, printer drift is generally impossible to model or predict because it depends on too many factors, both external and internal (e.g., temperature, humidity, printer age, etc.).
Printer drift has usually been solved by periodically recalibrating the printer. Printer calibration involves printing a set of test patches where the output is known assuming that the printer is in the reference state. The printed patches are then measured and compared to the known values for the reference state of the printer to determine a model of the printer drift. This model is then used to adjust the transfer curves (e.g., color conversion models) used for printing such that subsequent output can be corrected to the same as or close that of the printer in the reference state. Most printer manufacturers offer various calibration techniques to customers. For example, the “InfoPrint Manager” contains a halftone calibration system that allows the user to print test patches, measure the patches using an optical densitometer, and then recalibrate the printer using a single measurement set. This system is generally based on a single paper type.
To fully support cutsheet printers, a calibration system generally accounts for each print job using a variety of different paper types with each paper type behaving quite differently from another. Certain paper types may also not be available or known to a printer manufacturer, so a calibration system should allow end users to support any new paper type which complicates printer calibration. One manner of supporting multiple paper types includes assuming that papers are not that different from one another and using a transfer curve that is similar to the paper being printed to. Another manner includes assuming that enough information about a particular paper variation can be recovered by analyzing an International Color Consortium (ICC) profile of each paper without providing explicit paper management. Unfortunately, however, the paper characteristics cannot be fully recovered from ICC Profiles. In yet another manner, a reference paper is printed and measured for continuous use in the calibration. To calibrate other papers, a calibration system records the relationship of each paper and the reference paper and that relationship is then used to generate the calibration for the other papers. However, this method is not entirely accurate because it does not evaluate the actual paper in use. Accordingly, there exists a need to calibrate a printer for a variety of paper types that assures the paper being printed has an accurate transfer curve.